Revealing the reversible solid-state electrochemistry of lithium-containing conjugated oximates for organic batteries

In the rising advent of organic Li-ion positive electrode materials with increased energy content, chemistries with high redox potential and intrinsic oxidation stability remain a challenge. Here, we report the solid-phase reversible electrochemistry of the oximate organic redox functionality. The disclosed oximate chemistries, including cyclic, acyclic, aliphatic, and tetra-functional stereotypes, uncover the complex interplay between the molecular structure and the electroactivity. Among the exotic features, the most appealing one is the reversible electrochemical polymerization accompanying the charge storage process in solid phase, through intermolecular azodioxy bond coupling. The best-performing oximate delivers a high reversible capacity of 350 mAh g−1 at an average potential of 3.0 versus Li+/Li0, attaining 1 kWh kg−1 specific energy content at the material level metric. This work ascertains a strong link between electrochemistry, organic chemistry, and battery science by emphasizing on how different phases, mechanisms, and performances can be accessed using a single chemical functionality.


• Dilithium p-Benzoquinone dioxime (Li2-BQDO)
p-Benzoquinone dioxime (276 mg, 2 mmol) was dissolved in anhydrous THF (6 ml), and solid lithium hydride (32 mg, 4 mmol) was added to the solution under stirring. The reaction (performed in an Argon filled glove box) was allowed to stir for 48 h at room temperature. The reaction mixture was afterwards poured into 6 mL of anhydrous diethyl ether to precipitate the product, followed by centrifugation and washed with a copious amount of diethyl ether. The product (Li2-BQDO) was collected and dried under vacuum at 70 °C for 2 h and 200 °C for 12 h.

• Dilithium diphenylglyoxime (Li2-DPGO)
Diphenylglyoxime (240 mg, 1 mmol) was suspended in a mixture of anhydrous tert-butyl alcohol (5 ml) and MeOH (5 ml), and lithium tert-butoxide (160 mg, 2 mmol) was added to the suspension under stirring. The reaction (performed in an Argon filled glove box) was kept for 24 h at room temperature. The light brown precipitate was then filtered and washed with diethyl ether (10 mL) 3 times. The product (Li2-DPGO) was collected and dried at 60 °C, then at 200 °C for 2h under vacuum giving a yield of 90%. To a suspension of 9,10-phenanthrenedione (2.08 g, 10 mmol) in EtOH (50 ml), a EtOH (30 ml) solution of NH2OH×HCl (1.63 g, 25 mmol) and pyridine (2.03 ml, 25 mmol) was added. The resulting orange suspension was refluxed for 22 h, during which the solids dissolved, and the suspension turned to clear yellow solution. The resulting solution was cooled, filtered, and left for slow evaporation at room temperature. After 24 h, a yellow microcrystalline solid of the dioxime was formed, which was collected by filtration, washed with cold EtOH, and dried under vacuum for 24 h. The yield was 90%. The lithiation was performed by suspending H2-PADO (238 mg, 1mmol) in MeOH (5 ml). To the stirred suspension, lithium methoxide (76 mg, 2 mmol) was added. The reaction (performed in an Argon filled glove box) was kept for 24 h at room temperature. The resulting product was obtained by adding an excess amount of anhydrous diethyl ether (30 ml) to the solution. The product (Li2-PADO) was collected by filtration, and dried at 60   To a solution of phloroglucinol dihydrate (10 g, 0.06 mol) in methanol (100 mL), 0.5 ml of acetic acid was added. The suspension was cooled to 0 °C, and isopropyl nitrite (22 mL, 0.214 mol) was added dropwise within 1h, while stirring and keeping the temperature of reaction at 0°C. The mixture was stirred for another 0.5 h at room temperature. After that, the solvent was evaporated in vacuo at 35 °C of the water bath. The crude product was suspended in diethyl ether, a solid was filtered off, washed with diethyl ether, and dried. The material was used for the next step without further purification (52).
The 2,4,6-Trinitrosobenzene-1,3,5-triol (12.8g, 0.055 mol) obtained above was solubilized in 200ml of methanol to which 3.84g of hydroxylamine hydrochloride was added. The red solution was kept at room temperature for 30 days, afterwards, the white precipitate was filtrated, washed with water, methanol and dried at room temperature. Yield: 11.5 g of H4-TMTO, 65%. 100 mg (0.31mmol) of H4-TMTO was suspended in methanol (9ml) and lithium methoxide (48.5 mg, 1.27mmol) was added to the solution under stirring. The reaction (performed in an Argon filled glove box) was kept for 24 h at room temperature. Then, anhydrous diethyl ether (30 ml) was added to the reaction mixture, resulting in the precipitation of a white product, followed by filtration and washed with diethyl ether 4 times. The product (Li4-TMTO) was dried at 120 °C for 12h under a vacuum to remove free diethyl ether and methanol. Yield: 80.5mg, 75%.
-1 H NMR (300 MHz, MeOD): δ = 3.32 (6H, s, OMe) ppm. To a stirred solution of 1.38 g p-benzoquinone dioxime (10 mmol) and 0.84 g sodium hydroxide (21 mmol) in 30 ml water, 30 ml of 5.5% sodium hypochlorite (excess amount) was slowly added. The solution turned yellow, and a yellow precipitate gradually formed. Stirring was continued for 1h. The precipitate was filtered and washed with water 3 times and acetone, followed by drying under vacuum at RT to form PNND, with a yield of 65%.

Crystal structure determination by X-ray powder diffraction method.
The crystal structure of PNND was solved from the powder XRD data. Powder X-ray diffraction (PXRD) patterns were collected on a STOE Stadi P diffractometer in transmission geometry equipped with a Cu anticathode (Kα radiation, λ = 1.540600 Å, operating at 50 kV − 40 mA). For the structure determination from PXRD, we followed the following steps: pattern indexing, space group determination, structure solution and the Rietveld refinement. The indexing procedure was carried out in EXPO2014 (54), and FullProf suite (55) using NTREOR, DICVOL. By these indexing procedures a monoclinic system was obtained with high figure of merit (F.O.M.). Space group determination, structure solution and Rietveld refinement were performed in EXPO2014. Supplementary Figure S14. FTIR analysis survey for the synthesis of Li2-BQDO, coupled to air and moisture stability analysis.

Supplementary
The formation of lithiated product (Li2-BQDO, orange curve) is confirmed by the disappearance of the weak broad band around 3000 cm -1 attributed to the hydroxyl groups of H2-BQDO (black curve). After lithiation, the broad -OH band completely disappears (orange curve). Comparative FTIR spectra of pristine and dry air-exposed (for 48h) samples of Li2-BQDO (blue curve) and water vapours exposed (for 48h) Li2-BQDO (pink curve). The materials show identical FTIR signatures after dry air-exposure without any sign of decomposition, indicating excellent oxygen tolerance. For water vapor exposed Li2-BQDO (pink curve), residual water or partial hydrolysis can be observed with the characteristic peaks still preserved after 48h of exposure. Figure S15. FTIR analysis survey for the synthesis of Li2-DPGO, coupled to air and moisture stability analysis.

Supplementary
The formation of pure Li2-DPGO (orange curve) after lithiation of H2-DPGO (black curve) is confirmed by the disappearance of weak and broad O-H stretch band between 3200-3300 cm -1 . Comparative FTIR spectra of pristine and dry air-exposed (for 48h) samples of Li2-DPGO (blue curve) and water vapours exposed Li2-DPGO (pink curve, for 48h). The materials show identical FTIR signatures after dry air-exposure without any sign of decomposition. For water vapour exposed Li2-DPGO (pink curve), residual water or partial hydrolysis can be observed with the characteristic peaks still preserved after 48h of exposure. Figure S16. FTIR analysis survey for the synthesis of Li2-DMGO, coupled to air and moisture stability analysis.

Supplementary
The formation of lithiated product (Li2-DMGO, red curve) is confirmed by the disappearance of the weak broad band between 3000-3400 cm -1 attributed to the hydroxyl groups of H2-DMGO (black curve). Comparative FTIR spectra of pristine and dry air-exposed (for 48h) samples of Li2-DMGO show identical FTIR signatures after dry air-exposure without any sign of decomposition (oxidation nor hydrolysis). For water vapor exposed Li2-DMGO (pink curve), residual water or partial hydrolysis can be observed with the characteristic peaks still preserved after 48h of exposure. Figure S17. FTIR analysis survey for the synthesis of Li4-TMTO, coupled to air and moisture stability analysis.

Supplementary
The formation of lithiated product (Li4-TMTO, orange curve) is confirmed by the disappearance of the two broad bands between 3000-3500 cm -1 attributed to the hydroxyl group of H4-TMTO (black curve). Comparative FTIR spectra of pristine and dry air-exposed (for 48h) samples of Li4-TMTO show identical FTIR signatures (blue curve) after dry air-exposure, without any sign of decomposition (oxidation nor hydrolysis). For water vapor exposed Li2-DPGO (pink curve), residual water or partial hydrolysis can be observed with the characteristic peaks still preserved after 48h of exposure. Figure S18. FTIR analysis survey for the synthesis of Li2-PADO, coupled to air and moisture stability analysis.

Supplementary
The formation of lithiated product (Li2-PADO, orange curve) is confirmed by the disappearance of the broad band between 3000-3500 cm -1 attributed to the hydroxyl group of H2-PADO (black curve). Comparative FTIR spectra of pristine and dry air-exposed (for 48h) samples of Li2-PADO show identical FTIR signatures after dry air-exposure without any sign of decomposition (oxidation nor hydrolysis) proving stability under dry conditions as well as high oxidation potential (> 2.91 V vs. Li + /Li). For water vapour exposed Li2-PADO (pink curve), residual water or partial hydrolysis can be observed with the characteristic peaks still preserved after 48h of exposure.

Supplementary Figure S19. Crystal structure analysis of PNND.
Experimental X-ray powder diffraction pattern (black dotted line) compared with the Rietveld-refined profile (red curve) and the difference (blue curve) for PNND. Figure S20. Crystal structure of PNND polymer.

Supplementary
The crystal structure reveals that the polymer crystallizes in the monoclinic space group P21/ n (a = 6.394(2) Å, b = 11.322(4) Å, c = 3.7072(1) Å, β = 92.923(6) o , and V = 268.03(15) Å 3 , Table S1). The structure is similar to the reported CCDC structure with reference no. 2006601. Crystal structure shows that the compound polymerizes following the E configuration along the N=N. Therefore, the PNND polymer is made up of linear chains, which go parallel along the crystallographic a-axis and stack along the c-axis. In the polymeric chain, the atoms do not lie on the same plane, but they are distributed on different planes defined by the aromatic ring and the -ON=NO-connection. The dihedral angle formed by the two planes leads to a stair-like structural motif in the polymeric chains. For the neutral species (e.g. DPODO), high solubility was expected as the inter-molecular bonding in solid crystal are expected to be weak (no H-bonding, presumably mainly through VdW bonding) thus highly soluble in polar solvents.

Supplementary
For the PNND polymer, low solubility was expected, and also confirmed by measurements, while also keeping in mind the particular dissolution mechanism and species -the dissolution of PNND takes place via reversible dissociation of azodioxy dimers, generation of the dinitroso benzene intermediate, and solubilization of the later (as confirmed by GPC measurements).
The solubility of Li2-BQDO, and of other ionic lithiated oximate species, was in turn found to be higher than expected. However, this expectation was not based on quantifiable or established rules, but merely on general knowledge with organic battery materials, as well as literature survey. Typically, the ionic organic compounds have low solubility in battery electrolyte polar solvents, this being one strategy set forward for organic battery material stable cycling (3). This can be assigned to relatively strong binding of Li-cations to the dianionic organic center (or framework) and the solubility thus dependent on the solvation strength (polarity, dielectric constant) of the used solvent.
In the studied series of compounds, a trend can be indeed identified, with the di-anionic species (Li 2 -BQDO, Li 2 -DMGO, Li 2 -DPGO, Li 2 -PADO) having similar solubility in the range of 4-6 mM; whereas the tetra-anionic Li4-TMTO has considerably lower solubility (< 2 mM) which could be explained by a stronger ionic binding to Li-ions, or dissolution via ion pairs to compensate for. The rate capability of the Li2-BQDO electrode (active material mass loading of 4 mg/cm 2 ), measured at various C-rates, in a half-cell configuration, and in 7 M LiTFSI in EC/DMC used as electrolyte. The electrode can still reach a high capacity, close to 300 mAh/g, at a high 1C-rate (corresponding to 1 electron exchange in 30 min). From the Normalized capacity vs. Potential graph (right panel), an increase in the polarization with the increase of C-rate can be observed, attributed to a series of factors including working or Li-metal counter electrode polarization, as well as low ionic conductivity of the high concentration electrolyte. A pure Ketjen Black electrode was assembled in 2-electrode cell using Li-metal as counter/reference electrode and cycled in similar conditions as the studied active materials. For instance, the current corresponding to the C-rate applied in the graph above (for pure Ketjen Black electrode) is equal to the current densities applied to Li2-BQDO electrode (Fig.  S22). a) The redox reaction of Li2-BQDO and of Li2-BQ, highlighting the difference in the polymeric and single molecule oxidation reaction products. b) Cyclic voltammetry curve of Li2-BQ 100 mM tetrabutylammonium perchlorate in DMSO) overlapped with CV of Li2-BQDO (100 mM LiCl in DMSO). The measured potentials were calibrated with a Fc + /Fc internal reference. The analysis shows higher redox potential of Li2-BQDO compared to Li2-BQ, with two pairs of redox waves (anodic peaks: -1.13/-0.55, -0.68/-0.5 V for Li2-BQDO and -1.85/-1.69, -0.9 V/-0.8 V for Li2-BQ vs. Fc + /Fc), indicating a two-electron stepwise process. c) Cyclic voltammetry curves of Li2-BQDO measured in two different potential windows: orange curve -full scan window, with the two cathodic processes observed, and narrow potential scan window (black curve) with only one pair of redox peaks noted. The redox pairs are noted with 1 and 2 and highlight the asymmetric anodic and cathodic reaction pathways of Li2-BQDO.

Supplementary Figure S27. Electrochemistry of dilithium-dimethylgyloxime (Li2-DMGO).
Charge-discharge profile of Li2-DMGO electrode material as measured in a Lithium half-cell. The measurement was done with cycling rate of C/10 in a 5 M LiTFSI in tetraglyme electrolyte, with an active material mass loading of 4 mg/cm 2 . The charge plateau is located at around 3.1V vs. Li + /Li, while the discharge plateau is around 2.1V vs. Li+/Li, resulting in a large polarization of nearly 1V. The material utilisation is of 1.5 electron exchange (for a theoretical of two electrons), explained by the high solubility and elution from the electrode of both Li2-DMGO (Table S6) and its oxidized form (3,4-dimethyl-1,2,5-oxadiazole 2-oxide, that is a liquid) phases.

Supplementary Figure S28. Galvanostatic Intermittent Titration Technique (GITT) plot of the Li2-DPGO -DPODO redox as measured in a Li half-cell configuration.
The test was performed with intermittent discharge/charge periods of 2 hours (rate of C/10) followed by relaxation periods of 1 hour. The data shows that the main contribution to the polarization originates during the oxidation process (Li2-DPGO à DPODO, over 500 mV with the equilibrium not reached within one hour). Longer relaxation time have been also applied to allow the equilibrium to be reached, yet the solubility resulted in cells failure. The reduction process (DPODO à Li2-DPGO) only contributes by ~150 mV to the overall polarization, with the equilibrium reached within 1 h of relaxation.

Supplementary Figure S29. Structural reorganization of Li2-DPGO and Li2-PADO during the oxidation process.
The aim of investigating the electrochemical performance of dilithium 9,10-phenanthrene dioximate (Li2-PADO, 4) was based on the assumption that the kinetically limiting step (and thus contributing to large polarization observed in galvanostatic cycling, Fig. 3 main text) is the E-Z isomerization step of (Li2-DPGO, 2). However, high large hysteresis of ~1V was also observed for Li2-PADO, indicating that the E-Z isomerization is not the main contributor to the cell polarization (Fig. 3c). The experimental results have thus been corroborated to calculations for the energy profile of the Li2-DPGO ⟷ DPODO conversion process (Fig. 3e, analysing both, linear and closed structure reaction pathways, and in both, gas and solid phases). Ex-situ FTIR analysis was carried out during the cycling of DPODO electrode (70% active material and 30% conductive carbon). Three different state-of-charge states were analysed: pristine (oxidized DPODO form), two electrons reduced (DPODO à Li2-DPGO, discharged to 1.5 V vs Li + /Li) form, and one full redox cycle (Charge to 4 V vs Li + /Li, DPODO à Li2-DPGO à DPODO). The analysis shows excellent reversibility, given the identical spectra between the pristine and full-cycled data. The full-discharged spectrum was also found similar to the one of the as synthesized Li2-DPGO sample (Fig. S15), corroborating the electrochemical redox mechanism between DPODO and Li2-DPGO. Figure S31. PXRD data of Li2-BQDO and PNND phases and analysis of Li2-BQDO-8MeOH crystal structure.

Supplementary
(a) PXRD of PNND polymer, of Li2-BQDO-8MeOH and of Li2-BQDO phases. The crystal structure of PNND polymer was solved from the PXRD data (Figs. S19 and S20) whereas the crystal structure of Li2-BQDO-8MeOH was obtained from single crystal data analysis. An interesting feature worth to be noted is that Li2-BQDO-8MeOH is crystalline (black curve), whereas removing MeOH at 150 °C results in an amorphous Li2-BQDO phase (blue curve). The amorphous nature of the Li2-BQDO is preserved even during electrochemical charge-discharge cycles (Fig. 4, Main text), implying an exotic <amorphous small molecule salt> to <crystalline polymer> cyclic electrochemical conversion. (b) The single crystals of Li2-BQDO-8MeOH were obtained by slow diffusion of diethylether into a solution of Li2-BQDO in methanol at -30 °C over a period of one month. The X-ray diffraction analysis shows that Li2-BQDO-8MeOH crystallizes in the monoclinic space group P 21/n (a = 7.0409(4) Å, b = 15.3351(10) Å, c = 11.1693(7) Å, β = 94.206 (6)°, and V = 1202.73 Å3. Each of the oximate function of the molecule binding two Li cations, which are solvated by four methanol molecules in a tetrahedral coordination.

Supplementary Figure S32. In-situ XRD analyses. a) Schematic illustration of cell construct for the in-situ XRD measurements. b) In-situ XRD survey of Li2-DPGO during electrochemical cycling in solid phase.
The as synthesized Li2-DPGO is poorly crystalline (alike most of the Li-oximate derivatives disclosed in this work). Upon oxidation (charge), the Li2-DPGO phase gradually vanishes, simultaneous to the appearance of a new phase assigned to DPODO (matching the PXRD pattern of the chemically synthesized single crystal of DPODO (38)). The forward and reverse reactions thus proceed through a simultaneous two-electron, bi-phasic mechanism, corroborating the formation of furoxan ring after oxidation of Li2-DPGO (Fig. S29). Upon continuous cycling, the formed Li2-DPGO phase becomes less crystalline.

Supplementary Figure S33. Structural analysis of cycled electrodes.
SEM images of pristine PNND and Li2BQDO electrodes as well as after different cycling sequences. Li2-DPGO 5 × 10 -13

Supplementary
Li2-DMGO 1 × 10 -13 Li2-PADO 1 × 10 -12 Li4-TMTO 2 × 10 -12 We conducted tests to measure the electronic conductivity at room temperature of the lithiated oximates described in this study. The tests were carried out on compressed powder samples (7 mm in diameter), placed between stainless steel rods and subjected to an applied pressure of 3T. We used two-probe d.c. current-voltage technique to measure conductivity. The estimates conductivities are in the 10 -12 -10 -13 range, indicating insulating phases. The electrochemically synthesized PNND was prepared by galvanostatic cycling (10 full cycles) of Li2-BQDO, stopping the cycling in fully charged (oxidized) state, disassembling the cell, washing the positive electrode with DMC, followed by drying.

Gel Permeation Chromatography (GPC) analysis of soluble PNND species
The PNND has low solubility in common solvents. Only partial solubility was observed in DMF and DMSO (Table S6) after long sonication. After this step, the DMF dispersion was centrifuged and the supernatant was filtered through a 0.2-micron filter before GPC analysis. The GPC was performed on an Agilent gel permeation chromatography (GPC) system equipped with an Agilent 1100/1200 pump (25°C; eluent: DMF, 2.5mM of NH4PF6; flow rate: 1 mL/min). The estimated M are relative masses since these are determined with respect to polystyrene standards which have different hydrodynamic and elution volumes as compared to PNND, which is primarily composed of monomers and small-chain oligomers.
The analysis shows that the soluble species are of low M, in the range of the molecular weight of the 1,4-dinitrosobenzene monomer (136,11 g/mol), with a series of peaks with lower intensity and corresponding to integer multiples of the main peak MW. The low polydispersity (1.11 -1.12) for a potentially non-controlled polymerization process is also an indication that the species resulting from the solubilization of PNND are monomers or a few units (2)(3)(4)(5) oligomers.
The solubilization via depolymerization can be explained by the low activation energy for the dissociation of azodoxy dimers (20-30 kcal/mol), implying that this covalent bonding can DFT Calculation Section.

Computational setup.
DFT was performed for both gas-and solid-phase systems. The gas-phase simulations were performed with the GAMESS package (56). Total energies for each of the oxidation states, Etot were computed for the relaxed structures using the Minessota M11/M15 exchangecorrelation functionals (57) and the 6-31G** Pople basis set. Solid-state calculations were performed using SIESTA (58) which uses norm-conserving pseudopotentials and LCAO representation of the wavefunction. The exchange correlation functional used was PBE for solid (PBEsol (59)); this choice being known to produce reliable geometric structures for molecular crystals bulk states (60). In order to take into account the van der Waals interaction, we applied the Grimme's corrections to PBEsol (61).

Geometric models.
Gas phase calculations -were performed for two structures: closed and linear one ( Figure S35). In all cases, the results are reported after performing a structural relaxation up to a gradient of 0.01 eV/Å. Figure S35. Ball-and-stick representation for closed (left) and linear (right) structures of DPODO and DPGO 2in gas phase.

Supplementary
For the solid-phase calculations we start out investigations for two types of packing (i.e. for closed and linear structures, respectively). We used two types of computational models: in the first one, the structural relaxation is applied only to atoms in the cell (experimental geometry of the cell is preserved); second, the structural relaxation of the atomic positions and cell parameters is performed. For all models the structural relaxation was conducted to attain a gradient of less than 0.05 ev/Å. Results for the relaxed cells are given in Table S7. To investigate the insertion of Li in the structures we used supercell models with 2×2×2 unit cells, allowing more freedom to Li-ions (i.e. with respect to periodic boundary conditions) for both close/linear structures.

Supplementary
We calculated three models corresponding to a gradual insertion of Li-ions into the organic bulk: no Li atoms included, 50% of the maximum amount of Li atoms (i.e. 8 atoms in each super-cell i.e. a single Li atoms per molecule) and 100% of Li atoms (i.e. 16 Li atoms per supercell i.e. 2 Li atoms per molecule). The initial guess for the positions of Li atoms was produced by generating random positions at distances between 2.8 and 2.5 Å around the oxygen atoms, followed by full relaxation up to 0.05 ev/Å. As an example, the relaxed structures for 16 Li atoms in the two types of supercells in given in Figure S35.
The effect of Li insertion (i.e. 100% of Li atoms in the structure, that is 16 atoms/supercell) upon the cell parameters is presented in Table S9. As a qualitative comment we note that the effect of Li insertion upon the volume change of the unit cells is opposite for the two structures: for closed structure the volume is increasing in presence of Li, while for the linear one the volume is decreasing.  2 Results and Discussions.

Geometric structures.
The statistical distribution of selected bond lengths in various states was monitored by using histograms of interatomic pair distances. Since the system is periodic, not all the pairs are properly considered (i.e. the bonds at the border of the periodically repeated cell); the analysis on larger super-cells was thus performed, and then re-normalizing the total number to the cell used in calculations. Non-integer numbers in the histogram indicate atoms that are not properly taken into account due to periodic boundary conditions.
• à Li-O and Li-N pairs: A total of 100 steps between 1 Å and 5 Å was used to build the histograms, with the results for closed and linear structures being given in Figures S36 and  S37, respectively. It can be noticed that for closed structure ( Figure S36 • à C=N bond: To investigate the C=N bond evolution upon redox conversion, a smaller step in the histogram (0.002 Å) was used, since the C=N is a significantly stronger bond. The results for the two structures and for the three Li concentrations (i.e. 0%, 50% and 100%) are presented in Figure S38. In the absence of Li (no distortions of the structure) the number of corresponding C=N bonds in the histogram is obtained. For the linear structure (top), in absence of Li, sixteen C=N bonds with lengths around 1.344 Å (i.e. all bonds are equal in the absence of Li) are counted. For the closed structure (bottom), two groups of eight C=N bonds, with lengths around 1.342 Å and 1.352 Å, respectively, are noted -corresponding to the two types of C=N bonds in the closed structure.
The presence of 50% of all Li atoms leads to a random distribution of the C=N bonds for both structures. Indeed, the orange histogram indicates an almost uniform distribution of the bonds, which is the indication that no dominant structure is present in the two structures (linear or closed one).
For the fully saturated structure (100% Li), we note that in the closed structure the presence of a first Li atom has the effect that all C -N distances tend to increase: all values are superior to 1.34 Å (i.e. the bond length in absence of Li), with about ten bonds counted for 1.36 Å. For the C=N bond, this represents a weakening of the chemical bond in presence of Li atoms (longer distance, weaker bond). For the linear structure the effect is opposite, with many bonds centred around 1.33 Å (inferior to 1.345 Å in absence of Li). This indicates stronger bonding in presence of Li for the linear structure. Figure S39. Histogram of the C-N distances (nitrogen represented with green in the inset of the pictures, carbon with blue) as function of Li concentration for the two structural models presented in Figure S35. Top values-linear structure; bottom values -closed structure. It can be noted that presence of Li (yellow and purple boxes) has an opposite effect on the C-N bond length in the two structures: smaller (1.333 Å) for linear configuration, and larger (1.360 Å) for the closed configuration.

Summary of the geometric properties analysis:
The impact of Li atoms on the molecular configuration for the two structures in solid phase (i.e. closed and linear) has an opposite effect, as follows: • The volume of the elementary cell: linear structure has a lower volume of the unit cell, while the closed one has a larger volume of the unit cell, upon the insertion of Li.
• Strength of C=N bond: in the linear structure a shortening of the bond is brought by the presence of Li, while for the closed configuration, the bonds are longer (i.e. weaker).
• To note that the weakening of the C=N bond in closed structure leads to a complete breakup, so that the structure is not stable in presence of Li.
• Finally, the statistical analysis of the Li-O / Li-N distances reveals that Li is coordinated predominantly to O in the closed configuration, while for the linear one a similar statistical trend of Li-O and Li-N bonds is observed.

Energetic stability.
The total energy of the systems in bulk state and as free molecules, for each oxidation state was calculated. Energetic diagram in gas-phase of the closed/linear structures is represented in Figure S39, for relaxed molecules bearing total charges 0, -1 and -2. The calculations show that the closed structure is not stable for -2 charge (i.e., after structural relaxation at -2 charge, the closed structure is transformed into the linear one). It can be concluded that in gas phase, the most stable structure is the structure with total charge Q=-1, while the presence of the second electron is destabilizing the structure.
The diagram for total energy of the bulk states with different amounts of Li atoms inserted in the structure (i.e. 0 %, 50% and 100% of the total 16 atom/supercell) is given in Figure S40. We note that the presence of Li leads to an energetic stabilization of the structures. If no Li atoms are present, the closed structure is more stable, while in presence of 100% Li atoms the linear one is more stable. In the intermediate case (only half of Li atoms are insert) we found structures with energies that are close to each other (around 0.5 eV difference).
To be noted that the results on energetic stability corroborate those on the volume cell calculations in presence of Li. Indeed, for the linear structure we found that in presence of Li the cell volume diminishes, indicating a more efficient packing, while the total energy of the system shows an important energetic stabilization in this case.
The overall energy difference per Li atom (difference between the most stable structure in the two redox states / molecule / Li atom) is 0.67/0.72 eV for models with experimental / relaxed cell parameters, respectively.